When an external temperature is low, there occurs a problem that water generated in a fuel cell system freezes after the stop of the system, to break down pipes, valves and the like, and in view of such a problem, a method is suggested in which a scavenging process is executed to discharge remaining water externally from a fuel cell at the stop of the system.
However, in a case where the scavenging process is performed every time at the stop of the system, the inherently unnecessary scavenging process is performed to dry the fuel cell more than necessary, which causes a problem that startup properties deteriorate all the more.
In view of such a problem, a method is suggested in which when the operation of the fuel cell system is stopped, the impedance of the fuel cell is measured to detect the water state of the fuel cell (i.e., the dry state of an electrolyte membrane), thereby controlling the scavenging process based on a detection result (e.g. see Patent Document 1).
[Patent Document 1] Japanese Patent Application Laid-Open No. 2002-246053